Dead Loads to be included in Drilled Pier design for Slab-on-Grade Foundation

[oengineer]

When designing a Bell & Shaft drilled pier for a warehouse with a mezzanine floor, is it too conservative to include the weight of the grade beam & pier caps in the dead load for the pier design? Is it enough to just include Roof & mezzanine floor dead load? My thoughts are that even though the grade beams is resting on the soil, the grade beam needs to be designed as spanning from pile to pile. Since the grade beam is spanning from pier to pier, the pier needs to be designed to include the weight of the grade beam and cap. The entire span of the grade beam is not included, just the tributary length of the grade beam effecting the pier. Any suggestions/ comments are appreciated.

 

[JAE]

I think the full span tributary width of the grade beam should be included.

If soil concerns are settlement and piers thereby necessary - the grade beams must eventually span all the way from pier to pier should the soils move down.

If soil concerns are expansive clay pushing upward, then the grade beams should have permanent voids constructed beneath and they then span all the way from pier to pier as well.

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[oengineer]

@JAE - Thank you for your response. I figured that the grade beam weight (along with the cap) should be included in the design of underreamed/bell piers. I am analyzing a drawing and when I add the weight of the grade beam and cap I am unable to obtain the same size piers on the drawing. The drawing calls for 12/36 & 12/30 piers for exterior columns with a max soil allowable bearing capacity of 3200 psf. My grade beam weight alone is 19.13 kips. The grade beam is 1.5' x 2.5' with a trib. length of 34' @ a corner (20' on one end and 14' on the other).

 

[JAE]

Take off the displaced weight of the soil where grade beams, caps and piers exist. That might help you.

Also - any load imparted onto the grade beam needs to be included also.

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[oengineer]

@JAE -I am performing manual hand calculations using a spread sheet. I am just adding up my Dead Load (Roof, Grade Beam, Cap) and my Live Load (Roof Live) and putting it into a spread sheet and checking for a 12/30 & 12/36 pier. I have not included the displaced soil from grade beam into my calculations. The pier I am analyzing is not supporting a second floor.

 

[JAE]

At 3200 psf your 36" diameter bell has 22.6 kips of capacity. That isn't much and for deeper belled piers I am a bit surprised at the 3,200 psf number. Are you sure this is a valid capacity?

In some cases for deep (30 ft. to 40 ft.) piers with bells we would work with something > 10,000 psf.

Otherwise I'm not sure why the numbers are so low. It may be that the original designer did not include the grade beam weight in the design, which probably isn't correct.


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[oengineer]

@JAE - The piers are drilled to a min. of 8'-0" below natural grade. This project is located in Houston,TX. I received a soil report for a site in Houston,TX for a different project with soil bearing of 3,900 psf (with a Safety Factor of 2). I also think the original designer did not include the weight of the grade beams.

So would excluding the dead grade beam weight cause structural/ foundation failure? Or does the safety factor provided in the allowable soil bearing pressure given by the geotech engineer take care of any failure?

 

[JohnnnyBoy]

@JAE "Take off the displaced weight of the soil where grade beams, caps and piers exist. That might help you."

I do agree with removing the soil weight of for the displaced section of the piers and caps although would the displaced weight of the soil where grade beams are poured be subtracted? Since the grade beam weight would only be imposed on the piles when the soil settles I don't believe the weight of the soil displaced by the grade beam should be accounted for since as the soil settles the entire weight of grade beam would be transferred to the pile.

Could another option be to model grade beam and pile and to assume the soil under the grade beam to be a spring. With the expected settlement of belled piles included for the calculations of the spring constant, this value may be easily obtained from the geotech on the project. I would suspect in many cases that the grade beam not be fully transferred to the piles and the settlement of the native soil under the grade beams be fairly low in comparison to the settlement of the piles themselves, although this would depend on exact site soil conditions.

I further agree with JAE that 3200 psf is pretty low for end bearing piles and there may poor soil conditions. If this is the case the gradebeam will need to accounted for.

 

[JAE]

JohnnyBoy - I think you are correct - the soil below the bell doesn't currently feel the weight of the soil away from the pier at the grade beams.



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[aeoliantexan]

From a geotech's perspective:

Houston has some pretty weak clays, so the 3200 psf may be appropriate.

The clays are often expansive, and that is the first consideration. If the soil may heave under the grade beams, void forms are necessary, and as JAE said, the grade beam weight will go to the piers.

If this warehouse will have live loads of several hundred psf on the slab-on-grade floor, the soil could settle out from under the grade beams. Again, the beam weight would go to the piers.

For a lightly loaded building, like a school, on non-expansive soil, with estimated pier settlements small, I might assume the grade beams act as their own footings. They need to be constructed like footings, on a trimmed, undisturbed bearing surface.

 

[oengineer]

@JohnnnyBoy - What would you consider a good end bearing for piles in order to not consider the grade beam weight? Would 3900 psf suffice?

 

[oengineer]

@aeoliantexan - So adding the weight of the grade beams to the pier is not to be done every time when designing pier foundations? Are you saying it is on a case by case basis? The projects I am working on are in the Houston area.

 

[JohnnnyBoy]

I currently work in Alberta therefore our expected bearing pressure may be drastically different. I would go with @aeoliantexan perspective on the local bearing pressure as he has experience in that area. I would further agree that it would be more of a case by case basis on whether or not grade beams should be included in the calculations. If there is any doubt on the manner then it should be considered to be on the safe side or further analysis on settlements and expected soil behavior will need to be completed.

 

[aeoliantexan]

I don't know what the codes may require; I was just addressing some of the factors. In some cases I have included the weight of pile caps or pier caps in the pile or pier loads, in other cases not, on the philosophy that when the cap is poured, the soil is carrying the weight, and when the column is loaded, the pier will settle a little and the cap will probably take some of the load rather than add to it. This is only true if the soil is stable, not subject to shrink/swell or deep-seated settlement.

A few things about this building seem odd:
22 kips design column load seems pretty light for a roof plus mezzanine.
3200 psf is on the order of a footing bearing pressure. Perhaps the designer is using piers to get below the depth of seasonal shrink/swell, not to get to stronger soil. Again, if shrink/swell is an issue, perhaps void forms under the beams are needed. I would also look at the floor design; is it allowed to float or connected to the grade beams?
Are the walls supported on the grade beams?

 

[oengineer]

@aeoliantexan - The walls are supported on the grade beam. The floor is connected to the grade beams.

 

[aeoliantexan]

What is your ground floor design live load?
If you have the geotech report, does it mention potential shrink/swell of the soil?